Axial flow turbomachinery typically includes one or more rotating assemblies or disks. Each disk contains a number of radially directed blades. Each such bladed disk is rotatable with respect to a stationary surface or shroud which circumferentially surrounds each disk. The radially outer end or tip of each blade forms a narrow gap or clearance with respect to the shroud. Ideally, such gap should not exist. However, in practice, the bladed rotor and concentric shroud do not form invariant and perfectly circular shapes. Various forces acting thereon create distortions. For example, temperature changes create differential rates of thermal expansion and contraction on the rotor and shroud which may result in rubbing between the blade tips and shrouds. In addition, centrifugal forces acting on the blades and structural forces acting on the shroud create distortions thereon which may result in rubs.
Such rubs result in deterioration of the blade tips and/or shroud surface thereby increasing the average gap, hereinafter referred to as tip clearance. Increases in tip clearance result in significant decreases in the gas turbine engine efficiency, and hence in fuel burned.
Generally, the blade tips, prior to assembly within the casing, may be shaped to within very narrow tolerances with respect to blade length affecting tip clearance. In contrast, casing out of roundness and eccentricities between the rotor and shroud axes are difficult to avoid especially during engine operation. Thus, during certain periods of engine operation the blade tips may contact the shroud in certain interference regions. If the blade tips are made sacrificial and are worn away by contact in such regions, the average tip clearance in the non-interfering regions increases thereby reducing engine efficiency. However, if the blade tip has an abrasive coating, the shroud may be cut away in the interfering regions and the gap in the non-interfering regions will not be affected.
In either situation, some wearing of the blade tips is inevitable. In order to accomodate blade rubs without deleterious effects of rubs on blades, it is known to utilize "squealers" on the radially outer end of the blade. The "squealers" typically are elongated extensions of the airfoil and are essentially a long thin fin which cracks easily and is difficult to cool.
As noted above, it is also known to use abrasive coatings on blade tips. For example, U.S. Pat. No. 4,232,995-Stalker et al and U.S. Pat. No. 4,390,320-Eiswerth disclose blade tips with abrasive coatings. Such blade tips have proven effective for their intended purpose. However, assuring a good bond between the abrasive coating and the blade tip is critical. Blade tip rubs tend to occur quickly and produce a shear force on the coating. Prior art blades rely upon the strength of the bonding between the abrasive coating and the blade tip to resist such forces.